1. Field of the Invention
This invention relates to an elevator, and more particularly to a braking device for an elevator provided with the function of preventing running of the cage with excessive speed on occurrence of abnormality such as free fall or some malfunction etc.
2. Description of the Related Art
In recent years, instead of the traction-type elevator wherein the cage is raised and lowered by winding-up drive using a conventional suspension rope, various types of self running elevators have been proposed wherein the cage runs freely within the ascending/descending path under drive provided by a linear motor.
Hereinafter a conventional self running elevator is described as a related art of this invention. Such a self running elevator has a construction wherein, as shown for example in Japanese patent disclosure number H2-261789, the primary coil and secondary conductor of a linear motor are provided facing each other with a small gap therebetween on the left and right side faces of the cage and the left and right inside wall faces of the ascending/descending path, so that the cage runs along guide rails within the ascending/descending path under the propulsive force generated between the primary coil and secondary conductor of the linear motor.
Self running elevators of this kind have various advantages as described below. Since a winding up mechanism and/or suspension rope are not employed there is no restriction on the length of the ascent/descent, so that the elevator can be employed in skyscrapers etc. Furthermore its transportation capabilities can be improved by the fact that more than one cage can run along a single ascending/descending path. Additionally, the need to provide a machinery room directly above the ascending/descending path is eliminated.
In the self running elevators of this type provided with no suspension rope, the linear motor generates a propulsive force exceeding the weight of the cage so as to raise or lower or stop the cage in the space within the ascending/descending path. But there is the risk that the cage could fall down under abnormal circumstances such as free fall or other malfunction etc. For this reason, safety measures to prevent the cage falling are an important problem.
The braking device disclosed in Japanese patent disclosure number H2-261789 referred to above has therefore been considered. Such a braking device has a construction wherein there are provided: a pair of left and right levers that are free to open and close, having brake shoes at their ends that clamp the guide rails; compression coil springs that bias these left and right levers such that the brake shoes at their ends pressure-contact the guide rails; and solenoids and links that act to open the left and right levers against these compression coil springs.
With such a braking device, during running of the cage, the solenoids are excited by applying voltage thereto, so that the left and right levers are actuated for opening against the compression coil springs by this magnetic force, thereby separating the brake shoes from the guide rails and releasing braking. In braking, voltage to the solenoids is cut off, causing the left and right levers to be actuated for closing by the spring force of the compression coil springs, so that the brake shoes are pressed against the guide rails and braking of the cage is effected by the frictional force that is then produced.
Since the self running elevator braking device described above employs a system wherein braking force is provided by pressure contact of the brake shoes with the guide rail, produced by the compression spring force, extremely powerful compression coil springs are required. Furthermore, in order to release the braking against these compression coil springs, the solenoids themselves must be of large size so as to provide an attractive force of around 1000 kg. This greatly increases the cage weight.